Noise Reduction for Overlay Control

ABSTRACT

The present disclosure provides a system. The system includes a metrology tool configured to collect overlay errors from a patterned substrate; and a controller module coupled to the metrology tool and configured to generate an overlay compensation from the collected overlay errors, wherein the generating of the overlay compensation includes identifying a portion of the overlay errors as a set of outliers, identifying inside the set of outliers overlay errors not due to reticle effects, thereby creating a set of noise, excluding the set of noise from overlay errors, thereby creating a set of filtered overlay errors, and calculating the overlay compensation based on the set of filtered overlay errors.

PRIORITY

This is a continuation application of U.S. patent application Ser. No.16/402,828, filed May 3, 2019, which is a divisional application of U.S.patent application Ser. No. 15/644,126, filed Jul. 7, 2017, which claimsthe benefits of U.S. Prov. App. No. 62/435,009 entitled “Noise Reductionfor Overlay Control,” filed Dec. 15, 2016, the entire disclosure ofwhich is herein incorporated by reference.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling down has also increased the complexity ofprocessing and manufacturing ICs and, for these advances to be realized,similar developments in IC processing and manufacturing are needed.

With small feature sizes in advanced technology nodes, control ofsemiconductor fabrication processes faces many challenges. In asemiconductor fabrication facility, monitoring the results of processsteps has become critical. Misalignment, lithography defects, and tooldrift can cause unsatisfactory results in a process even after a periodof time with satisfactory results. Overlay monitoring and controlbecomes crucial to minimize overlay errors. Non-systematicimperfections, such as dust particles, overlay mark asymmetry, oroverlay mark damages, may cause large overlay errors, often referred toas “noise.” Noise degrades accuracy in overlay control. It is desirableto have techniques that can reduce or filter out the noise duringprocess overlay control.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1A is a block diagram of a system for overlay monitoring andcontrol constructed in accordance with some embodiments of the presentdisclosure.

FIG. 1B is a schematic diagram of an exposing tool constructed inaccordance with some embodiments of the present disclosure.

FIG. 2 is a flowchart of a method of semiconductor fabrication processcontrol in accordance with some embodiments of the present disclosure.

FIG. 3 is a sectional view of a substrate constructed in accordance withsome embodiments of the present disclosure.

FIG. 4 illustrates a wafer map with overlay error vectors depicted ontop of a visual representation of the wafer in accordance with someembodiments of the present disclosure.

FIG. 5 shows a portion of a wafer map illustrating overlay correctionthat benefits from noise reduction in accordance with some embodimentsof the present disclosure.

FIG. 6 shows a portion of a wafer map containing outliers among overlayerror vectors.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. For example, if the device in the figures is turned over,elements described as being “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the exemplary term “below” can encompass both an orientation ofabove and below. The apparatus may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein may likewise be interpreted accordingly.

The present disclosure is generally related to overlay monitoring andcontrol to ensure alignment quality of one or more semiconductor layers,more particularly to noise reduction to increase overlay correctionaccuracy. Overlay monitoring and control are used at various stepsduring semiconductor manufacturing processes. For example, a metrologytool can measure overlay errors by performing an overlay measurement ona work piece (e.g., a semiconductor wafer). An overlay measurementgenerally indicates how accurately a first patterned layer aligns with asecond patterned layer disposed above or below it or how accurately afirst pattern aligns with a second pattern disposed on the same layer.The overlay error is typically determined with an overlay mark havingstructures formed on one or more layers. The structures may take theform of gratings, box-in-box, or other suitable shapes. If the twolayers or patterns are properly formed, then the structure on one layeror pattern tends to be aligned relative to the structure on the otherlayer or pattern. Otherwise, the structure on one layer or pattern tendsto be offset or misaligned relative to the structure on the other layeror pattern. The overlay error is the misalignment between thecorresponding structures.

Moreover, if a measured overlay error is unacceptable (e.g., out of apredetermined range), the overlay metrology may be used to alter one ormore parameters of the process such that additional wafers manufacturedby the process can have acceptable characteristics. For example,measured overlay errors may be fed into an analysis routine thatcalculates adjustable parameters, which may be used by a controller toimprove the alignment of the lithography tool used in the waferprocessing.

Therefore, it is desirable to measure overlay errors of a set of overlaymarks as accurately as possible. Inaccuracy in a given set of overlaymetrology measurements may arise from non-systematic imperfections. Suchnon-systematic imperfections may include dust particles on wafersurface, overlay mark asymmetry, and overlay mark damages in a materiallayer (e.g., damages caused by chemical-mechanical polishing). Thesenon-systematic imperfections may lead to relatively sizeableinaccuracies in the overlay measurement, with an appearance of aplurality of outliers among measured overlay errors. Such outliers arealso referred to as “noise” throughout the present disclosure. As aresult, it is desirable to provide a system and method suitable fornoise reduction in overlay monitoring and control, mitigating the impactof overlay measurement inaccuracies.

FIG. 1A is a block diagram illustrating a system 100 for implementing amethod for overlay monitoring and control according to various aspectsof the present disclosure. It is understood that other configurationsand inclusion or omission of various items in the system 100 may bepossible. The system 100 is exemplary, and is not intended to limit thedisclosure beyond what is explicitly recited in the claims. The system100 includes a controller 102, a patterning tool 106, and a metrologytool 108. The controller 102 is coupled with the patterning tool 106 andthe metrology tool 108. In some embodiments, the patterning tool 106 isalso coupled with the metrology tool 108.

The controller 102 controls the operation of the patterning tool 106 andretrieves metrology readings from the metrology tool 108 for furtherprocessing. In some embodiments, the controller 102 includes an overlaycontrol module 110. The overlay control module 110 is able to generateoverlay compensation, determine a set of adjustable parameters based onoverlay measurement results, and feed forward to the patterning tool 106for alignment adjustment. The overlay control module 110 may performstatistical analysis of overlay errors. For example, the overlay controlmodule 110 may calculate overlay correction maps using overlay errorsfrom previous measurements from a single or a plurality of wafers in onelot, or in a plurality of lots. The overlay control module 110 mayfurther remove noise from overlay errors before generating overlaycorrection maps based on applying one or more noise filteringalgorithms, thereby improving overlay correction accuracy. In someembodiments, the controller 102 is implemented in a computer, includinga processor, memory, and an interface. The controller 102 may beimplemented in a single computer or a distributed computer, and connectsto various components of the patterning tool 106 and the metrology tool108, including but not limited to the connections shown in FIG. 1A. Insome embodiments, the controller 102 includes one or more softwareprograms for processing metrology data, such as overlay errors.

The system 100 may also include an integrated circuit (IC) designdatabase 104 coupled to the controller 102. The IC design database 104is designed to store and manage IC design layout data. In someembodiments, the IC design database 104 includes a plurality of ICdesign layouts which include IC layout patterns. The IC layout patternswill be transferred onto a semiconductor wafer using a mask (or reticle)and the patterning tool 106 to form various circuit components on thewafer. The circuit components may include transistors, capacitors,resistors, and/or metal lines. In some embodiments, the design databasemay be integrated in the controller 102.

In various embodiments, a plurality of metrology data is collected fromdifferent positions on the wafer. The controller 102 may use information(e.g., process parameters) from the design database 104 and thepatterning tool 106 to generate a metrology sampling plan, determiningthe number and positions of overlay marks to collect metrology data.These overlay marks are patterned on the mask and then transferred to amaterial layer (e.g., a resist layer) on the wafer during patterning.Each metrology data includes information such as overlay error and/orother measureable parameters, such as widths, heights, criticaldimensions, and thickness. In some embodiments, overlay errors aremeasured by comparing misalignment of overlay marks in a resist layerwith overlay marks formed in a previous patterned material layer. Insome embodiments, overlay errors are measured by comparing the positionsof the predetermined overlay marks and the corresponding marks on aresist pattern formed during exposure. Overlay errors collected from thewafer can be combined to form an overlay error map. An overlaycompensation, also referred to as overlay correction, may be generatedbased on the overlay error map to adjust some adjustable processparameters of the patterning tool 106, so that overlay errors may bereduced in future patterning processes. For example, an overlaycorrection may indicate a linear model showing a linear increasing ordecreasing trend along a certain direction. The patterning tool 106 maybe adjusted to compensate for the previously discovered trend.

The patterning tool 106 of FIG. 1A is further described in detail withreference to FIG. 1B in a schematic view. The patterning tool 106 isoperated during a procedure of patterning a substrate 160. In thepresent embodiment, the patterning tool 106 is a lithography system(also referred to as the lithography system 106) and the substrate 160is a semiconductor wafer, such as a silicon wafer, or other suitablewafer to be patterned. The lithography system 106 is operable to exposea resist layer coated on the substrate 160. In some embodiments, thelithography system 106 includes a radiation source (illumination source)152 to generate radiation energy (or radiation beam) to expose theresist layer. The radiation source 152 may be any suitable light source.In various embodiments, the radiation source may include a light sourceselected from the group consisting of ultraviolet (UV) source, deep UV(DUV) source, and EUV source. For example, the radiation source 152 maybe a mercury lamp having a wavelength of 436 nm (G-line) or 365 nm(I-line); a Krypton Fluoride (KrF) excimer laser with wavelength of 248nm; an Argon Fluoride (ArF) excimer laser with a wavelength of 193 nm; aFluoride (F₂) excimer laser with a wavelength of 157 nm; or other lightsources having a desired wavelength (e.g., below approximately 100 nm).In another example, the light source has a wavelength of about 13.5 nmor less.

The lithography system 106 also includes an optical subsystem 170 thatreceives radiation energy from the radiation source 152, modulates theradiation energy through a mask 156, and directs the radiation energy tothe resist layer coated on the substrate 160. In some embodiments, theoptical subsystem 170 includes an illumination module 154 and aprojection module 158. In some embodiments, the optical subsystem 170 isdesigned to have a refractive mechanism. In such situation, the opticalsubsystem includes various refractive components, such as lenses.

The illumination module 154 may include a single lens or a lens modulehaving multiple lenses and/or other lens components. For example, theillumination module 154 may include microlens arrays, shadow masks,and/or other structures designed to aid in directing the radiationenergy from the radiation source 152 onto the mask 156 that defines apattern to be transferred to a resist layer coated on the substrate 160.

The mask 156 is loaded and secured on a mask stage 166. The mask stage166 is designed and configured to be operable for translational androtational motions. The mask 156 has a pattern to be transferred to thesubstrate 160. The pattern of the mask 156 may include a plurality ofpredetermined overlay marks used in the following overlay monitoring andcontrol process. In some embodiments, the mask 156 includes a substrateand a patterned layer formed on the substrate. In some embodiments, themask 156 includes a transparent substrate and a patterned absorptionlayer. The transparent substrate may use fused silica (SiO₂) relativelyfree of defects, such as borosilicate glass and soda-lime glass. Thetransparent substrate may use calcium fluoride and/or other suitablematerials. The patterned absorption layer may be formed using aplurality of processes and a plurality of materials, such as depositinga metal film made with chromium (Cr), or other suitable material, suchas MoSi. A light beam may be partially or completely blocked whendirected on an absorption region. The absorption layer may be patternedto have one or more openings through which a light beam may travelwithout being absorbed by the absorption layer. The mask 156 mayincorporate other resolution enhancement techniques such as phase shiftmask (PSM) and/or optical proximity correction (OPC).

The projection module 158 may have a single lens element or a pluralityof lens elements configured to provide proper illumination to the resistlayer coated on the substrate 160. Each lens element may include atransparent substrate and may further include a plurality of coatinglayers. The optical subsystem 170 may further include additionalcomponents such as an entrance pupil and an exit pupil to form an imageof the mask 156 on the substrate 160 secured on a substrate stage 162,which is capable of securing and moving the substrate 160 intranslational and rotational modes.

In other embodiments where the radiation energy is EUV energy, the mask156 and the optical subsystem 170 are designed to have a reflectivemechanism. In this situation, the optical subsystem 170 includes variousreflective components, such as mirrors, designed and configured toachieve the respective functions. The mask 156 has a reflectivestructure. In one example for illustration, the mask 156 includes asubstrate with a low thermal expansion material (e.g., TiO₂ doped SiO₂),a multiple reflective multiple layers (ML) deposited on the substratewhere the ML includes a plurality of film pairs (e.g.,molybdenum-silicon film pairs), and an absorption layer (e.g., atantalum boron nitride layer) deposited over the ML. The absorptionlayer is patterned according to an IC design layout.

Referring back to FIG. 1A, the metrology tool 108 is designed to collectmetrology data, including overlay errors, from predetermined locationsof a substrate. In some embodiments, the metrology tool 108 receives asemiconductor wafer with a resist pattern on it, performs an overlaymeasurement of the resist pattern to obtain overlay errors from overlaymarks at predetermined locations, and subsequently send measurementresults to the controller 102 for further processing, for example, inthe overlay control module 110.

The system 100 is described according to various embodiments. However,in various embodiments, the various modules of the system 100 may beintegrated together. For example, although FIG. 1A illustrates themetrology tool 108 separated from the patterning tool 106, the metrologytool 108 may be integrated into the patterning tool 106 in any suitableconfiguration. In some embodiments, the various modules of the system100 may be distributed in different locations and coupled togetherthrough intranet or internet. In some embodiments, various functions maybe built in different modules. For example, the overlay control module110 may be integrated in the controller 102 or in the metrology tool108.

FIG. 2 is a flowchart of a method 200 for fabricating a semiconductorstructure and overlay monitoring and control associated therewith,constructed in accordance with various embodiments. In some embodiments,the method 200 is implemented using the system 100 as shown in FIGS. 1Aand 1B. It is understood that additional steps can be provided before,during, and after the method 200, and some steps described can bereplaced, eliminated, or moved around for additional embodiments of themethod 200. The method 200 is an example, and is not intended to limitthe disclosure beyond what is explicitly recited in the claims.

The method 200 includes an operation 202 by patterning a work piece(e.g., a semiconductor wafer) by a patterning tool, such as thepatterning tool 106. The method 200 is thereafter described withreference to FIGS. 3, 4, and 5. FIG. 3 illustrates an exemplarysemiconductor structure 300 in a sectional view, constructed inaccordance with some embodiments. The semiconductor structure 300includes a substrate 310 and a material layer 320 on the substrate 310.The operation 202 forms multiple patterns in the material layer 320. Insome embodiments, the substrate 310 is a semiconductor substrate, suchas silicon substrate or a substrate having other semiconductor material(e.g., silicon germanium). Alternatively, the substrate 310 is aphotomask (mask or reticle), or other suitable substrates, such as athin-film-transistor liquid crystal display (TFT-LCD) substrate.

After patterning, the material layer 320 includes a main pattern 326having various main features. The main pattern 326 is defined accordingto features of an IC. The material layer 320 also includes variousoverlay marks 328 designed for overlay inspection and alignmentmonitoring. The material layer 320 may be a semiconductor material layer(e.g., silicon layer or silicon germanium layer), a dielectric material(e.g., an interlayer dielectric-ILD), a conductive material (e.g., metallayer or a doped polysilicon layer) or a resist material (e.g.,photoresist material) in various embodiments. The main pattern 326 mayinclude a doping pattern (e.g., various source and drain features formedin a semiconductor material layer), a gate electrode pattern (havingmultiple gate electrodes of polysilicon or metal), or an interconnectpattern having a plurality of conductive features (e.g., contacts, vias,or metal lines) in various embodiments. In some embodiments, thestructure 300 is a test structure (e.g., a test wafer or a controlwafer) patterned with mainly overlay marks 328 but without main pattern326 formed together. The material layer 320 is patterned by a suitabletechnology (e.g., lithography patterning). In the present embodiment,material layer 320 is a resist layer patterned by a lithography system.The resist layer is coated on the substrate 310 by a proper technique,such as spin-on coating. The resist layer is sensitive to the radiationbeam during a lithography exposing process and is resistive to asubsequent process (e.g., etching or ion implantation). The resist layermay be positive-tone resist or negative-tone resist. The operation 202may further include other processing steps, such as a thermal bakingstep to reduce solvent of the resist layer after the spin-on coating.

The semiconductor structure 300 is further described with a reference toFIG. 4. In the present embodiment, the substrate 310 is a semiconductorwafer (e.g., a silicon wafer). FIG. 4 illustrates a top view of a waferwith overlay errors depicted on top of a visual representation of thewafer. The top view is also referred to as a wafer map. A wafer goesthrough a plurality of IC fabricating operations, thereby formingmultiple chips (dies) on the wafer. Each chip includes a functionalintegrated circuit. A wafer further includes scribe lines definedbetween the chips. The chips are separated by dicing through the scribelines at a later fabrication stage.

From a top view, the wafer comprises many exposure fields 402. Duringlithography patterning, patterns defined on a mask are repeatedlytransferred to each field of the wafer. Particularly, the lithographypatterning process includes an exposing process in a proper mode, suchas step and scan. The mask is stepped to a field of the wafer and thecorresponding mask patterns are transferred to that field, and then themask is stepped to a next field and the mask patterns are transferred tothe field, and so on until the fields of the wafer are exhausted. Eachstep to a field requires machine-alignment. In some embodiments, eachfield 402 includes one chip or alternatively multiple chips. In variousembodiments, overlay marks are formed on each field. For example,overlay masks are formed on chip areas and/or on scribe lines. Thenumber overlay marks and the positions of overlay marks are designed tohave sufficient coverage of the wafer from field to field and/or fromchip to chip.

Referring to FIGS. 2 and 4, method 200 proceeds to operation 204 bycollecting metrology data on the current wafer, such as measuringdisplacement of overlay marks. Metrology data may be obtained from eachoverlay mark on the wafer, or alternatively only from overlay marks atselected positions predetermined by a metrology sampling plan. Metrologydata include information about overlay errors. Each overlay error can berepresented using a vector including a magnitude and a direction. Aplurality of overlay error vectors obtained can be depicted on the wafermap, forming an overlay error map. FIG. 4 also illustrates an overlayerror map showing overlay error vectors collected from the wafer 300,such as overlay error vectors 450 and 460. The difference betweenoverlay error vectors 450 and 460 will be described in more detailsbelow.

Factors causing overlay errors have become increasingly complicated withreduction of feature size in IC circuits. The principle factors are dueto systematic imperfections, including operational environment issues(e.g., vibration and/or temperature variation), stepper problems (e.g.,optical alignment, wafer clamping, and/or wafer table errors), reticleinaccuracy (e.g., reticle in-plane distortion and/or reticle clamping),process inconsistency (e.g., during exposure, development, and/oretching), and wafer distortion (e.g., wafer flatness and/or patternslip). Systematic imperfections are generally referred to as reticleeffects.

Reticle effects may appear systematically wafer-to-wafer or lot-to-lot.It is desirable to compensate reticle effects to improve alignmentquality in subsequent patterning operations. Reticle effects can becompensated to some extent by overlay correction. A correction model maybe utilized to determine overlay correction that is associated with oneor more adjustable process parameters of the patterning tool 106. Theoverlay correction is then applied to the patterning tool 106. In someembodiments, the overlay correction is associated with the opticalsubsystem 170 (such as the projection module 158 in FIG. 1B) and isapplied to adjust the optical subsystem 170 accordingly in order toeliminate or reduce the overlay errors during subsequent exposingprocesses. For example, the overlay correction is applied to theprojection module 158 such that the various optical components (such aslens or mirrors) are tuned with a configuration resulting in an imagingof the mask pattern to the wafer with decreased overlay errors. In oneparticular example, the compensation parameters are one or more opticalparameters, such as one or more coefficients of the Zernike polynomialassociated with the projection module 158.

Besides reticle effects, some other factors causing overlay errorsinclude non-systematic imperfections, such as large size dust particlesright over overlay marks, overlay mark asymmetry, and overlay markdamages on a wafer surface. As an example, an overlay mark in a materiallayer may have pattern damages during a chemical-mechanical polishingprocess, an etching process, or the like. As another example, an overlaymark may have an asymmetric feature after developing. Non-systematicimperfections occur randomly and do not carry consistent characteristicswafer-to-wafer or lot-to-lot. But they do cause inaccuracy in metrologytool's measurements, resulting in overlay error values that do notrepresent a true physical misalignment. In an overlay error map, overlayerror vectors due to non-systematic imperfections typically appear asrandom outliers among a set of vectors, also referred to as “noise” inthe present disclosure. Noise represents an unreliable overlay error.Still referring to FIG. 4, vectors 460 are examples of overlay errorvectors due to reticle effects, while vectors 450 are examples ofoverlay error vectors identified as noise (highlighted by circlingvector starting points for sake of clarity). According to the sources ofoverlay errors, corresponding metrology data (or overlay data) can becategorized as reticle effect data (non-noise data) or non-reticleeffect data (noise data).

Since overlay correction is calculated based on overlay errors. Thequality of a set of measured overlay errors is important for theaccuracy of overlay correction. Particularly, noise should be filteredout of the valid data. Otherwise, a correction calculated at thelocation of noise may cause even larger overlay error in subsequentprocesses. Further, noise usually appears as an outlier. To compensatean outlier, compromise usually has to be made to nearby locations ofnon-noise data that represent real reticle effects. Overall, this leadsto degraded correction accuracy.

FIG. 5 illustrates the effectiveness of applying a noise reductionaccording to some embodiments of the present disclosure by comparing twoprocesses, process (a) and process (b). Only a portion of a wafer map isillustrated in FIG. 5 for the sake of simplicity.

Process (a) illustrates overlay correction when no noise reduction isperformed. Contour line 402 represents the edge of a wafer. Overlayerrors are represented by a set of vectors. In this example, vectors 410and 412 are noise, for example, due to overlay mark damages incorresponding spot(s) in a resist layer. Therefore, they have unreliableoverlay error values. The other vectors (e.g., vector 414) representoverlay error caused by reticle effects. When the vectors 410 and 412are used together with other vectors by a correction model to calculatean overlay correction, the correction calculated would have a pooraccuracy. Moreover, overlay correction for nearby vectors would have tobe compromised for the correction of these outliers. For example, afteran overlay correction operation, an overlay correction result (referredto as an overlay residue) 416 corresponding to vector 414, whichoriginally has insignificant overlay error magnitude, becomes muchlarger than vector 414 itself.

Process (b) illustrates an overlay correction operation with vector 410and 412 identified as noise and removed from overlay data according tosome embodiments of the present disclosure before an overlay correctionmodel is applied. Without distortion from the noise, overlay correctionis more accurate and overlay residue is significantly reduced comparedwith process (a).

Referring to FIG. 2, method 200 proceeds to operation 205 by identifyingnoise from the overlay errors. In the present embodiment, the operation205 includes steps 206, 208, and 209 to identify noise by applying atleast two quantitative filtering processes. These steps may be operatedautomatically. Alternatively, the operation 205 may rely on a manualprocedure, such as a visual inspection conducted by an operator based onexperience, to pick out noise from the overlay errors. It should berecognized that the identifying and the subsequent removal of noisebefore an overlay correction calculation is advantageous as it putslarger weight in the calculation on those overlay data having a largerdegree of reliability, thereby improving the correction's quality.

Still referring to FIG. 2, Step 206 uses a quantitative analysis toidentify outliers among overlay data in each field based on a fieldscheme of the device 300. In some embodiments, the field scheme is thesame as the exposure field scheme. Since each quantitative analysis isoperated on overlay data collected in the same field, the quantitativeanalysis can be regarded as an “intra-field filtering.”

The overlay control module 110 may be programmed to automaticallyidentify outlier based on one of the conditions listed below or acombination thereof:

i) magnitude of overlay vectors exceeding a selected level;

ii) a selected percentage of the most outlying overlay data points(e.g., defining the largest 5% of overlay data as being outlying);

iii) a statistical distribution based on mean and standard deviation(e.g., defining overlay data with magnitude outside of 6-sigma as beingoutlying); and

iv) a relative magnitude variation.

In the present embodiment, the intra-field filtering compares themagnitude of a target data to that of a reference overlay data andconsiders an abrupt change in magnitude as being outlying. For example,the intra-field filtering may be executed by first sorting vectors ineach field according to corresponding magnitudes. The vector magnitudesare denoted from small to big (or big to small) as S₁, S₂, . . . S_(n).An overlay data is picked as a reference point, such as S_(n-k).Outliers are identified in conditions of

$\begin{matrix}{S_{n} > {S_{n - k} \cdot \left\lbrack {{\left( {C_{\max} - C_{\min}} \right)e^{{- R} \cdot S_{n - k}}} + C_{\min}} \right\rbrack}} & {{eq}\mspace{14mu} (1)} \\{S_{n - 1} > {S_{n - k} \cdot \left\lbrack {{\left( {C_{\max} - C_{\min}} \right)e^{{- R} \cdot S_{n - k}}} + C_{\min}} \right\rbrack}} & \; \\\vdots & \; \\{S_{n - k + 1} > {S_{n - k} \cdot \left\lbrack {{\left( {C_{\max} - C_{\min}} \right)e^{{- R} \cdot S_{n - k}}} + C_{\min}} \right\rbrack}} & \; \\{{{{where}\text{:}\mspace{14mu} k} = {1 \sim 5}},{C_{\max} = {5 \sim 10}},{C_{\min} = {0 \sim 5}},{R = {0.1 \sim {0.9.}}}} & \;\end{matrix}$

This quantitative analysis picks an overlay data as a reference point,such as S_(n-k). The value of k depends on how many points a user wantsto check in a field, while

S _(n-k)·[(C _(max) −C _(min))e ^(−R·S) ^(n_k) +C _(min)]

can be regarded as a threshold defined by the picked reference point.Then the quantitative analysis compares each overlay data in the samefield that has a magnitude larger than S_(n-k) to this threshold. If amagnitude is larger than the threshold, it means the correspondingoverlay data has a relatively large deviation in its vector magnitudefrom the reference overlay data and would thereby be identified as anoutlier. For example, if thirty overlay data are collected from a field(n=30), and the overlay control module 110 (or a user) determines thatfour points need to be checked (k=4) according to some empiricalknowledge, then S₂₆ (n-k=26) is selected as the reference point, andS₂₇, S₂₈, S₂₉, S₃₀ are compared individually to the threshold defined byS₂₆. If S₂₉ and S₃₀ are larger than the threshold, then these twooverlay data are flagged as outliers. While S₂₇ and S₂₈ have largervector magnitude than S₂₆, they are not identified as outliers in thisexample as their relative magnitude deviation from the reference overlaydata is not considered as an abrupt change. This operation issubsequently applied to a plurality of other fields to identify outliersin each corresponding field.

The operation 205 also includes a step 208 to identify reticle effectdata. With step 206, outliers are identified in each field. But anoutlier is not necessarily a noise. For example, similar outliers thatperiodically appear in a plurality of fields may be related to somesystematic imperfections and thus should be treated as useful datainstead of noise.

FIG. 6 illustrates a portion of a wafer map consisting of threeconsecutive fields 602, 604, and 606. Each field has a plurality ofoverlay error vectors, among which there are outliers. Step 206 mayidentify vectors 620 and 622 in field 602, vector 624 in field 604, andvector 626 in field 606 as outliers. But, these outliers may not all benoise. Vectors 622, 624, and 626 appear to be periodically appearing insimilar regions in these fields, which may be caused by some systematicimperfections. Therefore, vectors 622, 624, and 626 may be reticleeffect data instead of noise, while vector 620 may be considered as anoise.

Step 208 is configured to separate reticle effect data from outliersdetermined in step 206 by applying at least an inter-field filtering toexamine an overlay data's reoccurrence in a plurality of fields.

The appearance of overlay errors due to reticle effects can beattributed to both inter-field overlay errors and intra-field overlayerrors. As discussed above, a “field” refers to an exposure field on awafer each time machine-alignment is required. The inter-field overlayerror can be resulted from inter-field translation, inter-fieldsymmetrical and asymmetrical magnification, and inter-field symmetricaland asymmetrical rotation, such as mismatching problem between mask andwafer. Inter-field correction can be generated by applying aninter-field correction model. Inter-field correction compensates forwafer level deviation from a reference location (usually to the centerof the wafer). Thus, the inter-field correction can include acombination of one or more of the group consisting of translations,rotations, scaling, and non-orthogonality. The inter-field correctionapplied to each field may be different, and can be computed as a linearfunction of the location of each field on the wafer. In someembodiments, the inter-field correction model is given by:

D _(X) =T _(X)−(RW+N)·Y+S _(X) ·X+higher order terms

D _(Y) =T _(Y) +RW·X+S _(Y) ·Y+higher order terms  eq (2)

where:

(X,Y) are inter-field coordinate system, with respect to the center ofthe wafer,

D_(X), D_(Y) are inter-field overlay errors,

T_(X), T_(Y) are inter-field translations,

RW is wafer rotation,

N is non-orthogonality and

S_(X), S_(Y) are wafer scaling coefficients.

The intra-field overlay error can be resulted from intra-fieldtranslation, intra-field symmetrical and asymmetrical magnification, andintra-field symmetrical and asymmetrical rotation, such as misalignmentbetween a light source filter lens and a mask. An intra-field error ismeasured with respect to the center of an exposure field. Intra-fieldcorrection can be generated by applying an intra-field correction modelto each exposure field. In some embodiments, the intra-field correctionmodel is given by:

dx=T _(x)−(RS+RA)·y+(MS+MA)·x+higher order terms

dy=T _(y)+(RS−RA)·x+(MS−MA)·y+higher order terms  eq (3)

where:

(x,y) are intra-field coordinate system, with respect to the center of afield,

dx, dy are intra-field overlay errors,

T_(x), T_(y) are intra-field translations,

RS, RA are symmetrical and asymmetrical field rotations, and

MS, MA are symmetrical and asymmetrical field magnifications.

To identify reticle effect data, a filtering process that includes usingan inter-field correction model, an intra-field correction model, or acombination thereof may be applied. An overlay data is considered areticle effect data if the difference (denoted as “residue”) between thecorresponding overlay error (denoted as “R”) and a value calculated bycorrection model(s) is less than a threshold (K_(c)). For example, ifthe filtering process uses both an inter-field correction model and anintra-field correction model, an inter-field correction value iscalculated for an overlay data according to the inter-field correctionmodel and an intra-field correction value is calculated according to theintra-field correction model. The overlay data's residue is thereby thedifference between “R” and a sum of the inter-field correction value andthe intra-field correction value. This calculation may involve vectoroperations. The determination of a reticle effect data can be indicatedas follows:

residue=|R−(Inter-field correction value+Intra-field correctionvalue)|<=K _(c)  eq (4)

where:

K_(c)=1 nm˜7 nm in some embodiments.

In some embodiments, the inter-field correction model is a correctionper exposure (CPE) model. CPE allows different corrections to beprovided for different fields patterned by a scanner. In someembodiments, the intra-field correction model is an intra-field highorder process correction (iHOPC) model. The iHOPC model allows to modeldata with a high order polynomial model, including second order, thirdorder or even higher order terms. Those higher order terms arenon-linear. In one embodiment, the iHOPC model has an order selectedfrom 2 to 5. Step 208 compares magnitude corresponding to each overlaydata using equation (4) and thereby determines a set of reticle effectdata. Accordingly, a set of non-reticle effect data is also determinedwith overlay data not satisfying equation (4).

Still referring to FIG. 2, the operation 205 includes a step 209. Step209 examines whether an outlier identified in step 206 is actuallyidentified as a reticle effect data in step 208. If the answer is yes,then this outlier has physical meanings corresponding to real systematicimperfections and should not be flagged as noise. If the answer is no,then this outlier should be regarded as noise and be excluded fromfurther operations. The method 200 proceeds to operation 210 whichgroups overlay data that are not identified as noise into a set offiltered overlay data. Equivalently, the set of filtered overlay dataincludes outliers that are also identified as reticle effect datatogether with the non-outliers. Identifying noise from outliers can beperformed in each field or on a wafer level collectively.

With noise removed, the set of filtered overlay data has higher accuracyand reliability in representing overlay errors. A second round ofoutlier filtering may be optionally applied to the whole set of filteredoverlay data for finer noise reduction. The present embodiment includesan operation 212, which applies an inter-field correction model tofurther filter outliers. In this step, an inter-field correction valueis calculated for each overlay data according to the inter-fieldcorrection model. An overlay data is considered as an outlier if itsresidue as the difference between “R” and the respective inter-fieldcorrection value is larger than a threshold (K_(r)). This calculationmay involve vector operations. The determination of extra outliers canbe indicated as follows:

residue=|R-inter-field correction value|>K _(r)  eq (5)

where:

K_(r)=3 nm˜7 nm in some embodiments.

In some embodiments, the inter-field model is a high order processcorrection (HOPC) model with an order equal to or less than 5. HOPCmodel allows a higher order inter-field process correction. The higherorder terms are non-linear. The outliers identified in the second roundof outlier filtering are also considered as noise and excluded from theset of filtered overlay data.

Still referring to FIG. 2, the method 200 proceeds to an operation 214by determining overlay compensation according to the set of filteredoverlay data using an alignment model. The operation 214 includesvarious actions (sub-operations), leading to the determination of theoverlay compensation.

The alignment model is built up through a proper procedure. In someembodiments, the procedure includes selecting the compensationparameters, constructing a mathematical formula having the compensationparameters as a function of the overlay errors (or some extractedvariables of the overlay errors), and determining the coefficients ofthe formula according to manufacturing data. In some embodiments, thealignment model is a hybrid model that includes an inter-field model andan intra-field model. For example, a CPE model and an iHOPC model,alternatively a HOPC model and an iHOPC model.

Still referring to FIG. 2, the method 200 includes an operation 216 byperforming a compensation process to the patterning tool 106 accordingto the overlay compensation determined in the operation 214. During theoperation 216, the patterning tool (such as a lithography system) 106 isadjusted according to the overlay compensation. Parameters includingclamping force, tilting angle, translational shift, rotational shift ofthe wafer stage, and/or other various lithography exposing settings canbe adjusted. In some embodiments, the projection module 158 is adjusted.In some other embodiments, the wafer stage 162 is additionally oralternatively adjusted. In some other embodiments, other module of thelithography system is included in the alignment model and thecompensation process. For example, mask stage 166 may be included in thealignment model and the compensation process. In a more particularexample, the clamping force or tilting angle of the mask stage 166 maybe compensated. By the operation 216, the patterning tool 106 iscorrected to reduce the overlay errors in a feed forward manner. Forexample, the overlay compensation is determined based on the overlaydata from the current wafer, and subsequent exposing processes with thecorrected system can be applied to the same wafer or alternativelyanother wafer.

The present disclosure provides a method for a patterning process withoverlay monitoring and correction, where the overlay compensation isdetermined according to the predetermined overlay errors and is appliedto compensate the patterning system to enhance the overlay compensationaccuracy. The overlay compensation accuracy is enhanced by identifyingand filtering noise from overlay metrology data. The noise reductionprocedure includes the applying of both inter-field and intra-fieldfiltering models.

The embodiments of the present disclosure offer advantages over existingart, though it is understood that other embodiments may offer differentadvantages, not all advantages are necessarily discussed herein, andthat no particular advantage is required for all embodiments. Byutilizing the disclosed method, the full mapping and intra-field highorder process correction are achieved without decreasing the throughputof the lithography exposing process. The method also provides dynamicfeed forward control to reduce overlay errors, enhancing the overlayquality wafer by wafer and lot by lot.

Thus, the present disclosure provides a method in accordance with someembodiments. The method includes patterning a substrate by a patterningtool; collecting a plurality of overlay errors from a plurality offields on the substrate; identifying noise from the plurality of overlayerrors by applying a first filtering operation and a second filteringoperation that is different from the first filtering operation; groupingthe plurality of overlay errors that are not identified as noise into aset of filtered overlay errors; calculating an overlay compensationbased on the set of filtered overlay errors; and performing acompensation process to the patterning tool according to the overlaycompensation.

The present disclosure also provides a method for overlay monitoring andcontrol in accordance with some other embodiments. The method includesforming patterns on a wafer by a lithography system; collecting anoverlay error set from the patterns, wherein the overlay error setincludes a plurality of overlay error subsets according to a fieldscheme of the wafer; identifying outliers from each of the plurality ofoverlay error subsets; identifying noise from the outliers by applying afirst inter-field correction model and an intra-field correction model;forming a filtered overlay error set by excluding the noise from theoverlay error set; removing extra noise from the filtered overlay errorset by applying a second inter-field correction model; after theremoving of the extra noise, generating an overlay compensation from thefiltered overlay error set; and adjusting at least one parameter of thelithography system according to the overlay compensation.

The present disclosure provides a system for patterning and overlaycontrol in accordance with some embodiments. The system includes apatterning tool configured to form patterns on a wafer; a metrology toolconfigured to measure overlay errors associated with the patterns ineach field of the wafer; and a controller module configured to read theoverlay errors from the metrology tool, generate an overlaycompensation, and feed the overlay compensation to the patterning toolto adjust a parameter of the patterning tool. The generation of theoverlay compensation includes identifying noise from the overlay errorsby applying at least two filtering operations; grouping the overlayerrors that are not identified as noise into a set of filtered overlayerrors; removing extra noise from the set of filtered overlay errors byapplying another filtering operation that is different from the at leasttwo filtering operations; and after the applying of the anotherfiltering operation, calculating the overlay compensation from the setof filtered overlay errors.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A system, comprising: a metrology tool configuredto collect overlay errors from a patterned substrate; and a controllermodule coupled to the metrology tool and configured to generate anoverlay compensation from the collected overlay errors, wherein thegenerating of the overlay compensation includes: identifying a portionof the overlay errors as a set of outliers; identifying inside the setof outliers overlay errors not due to reticle effects, thereby creatinga set of noise; excluding the set of noise from overlay errors, therebycreating a set of filtered overlay errors; and calculating the overlaycompensation based on the set of filtered overlay errors.
 2. The systemof claim 1, further comprising: a patterning tool configured to form thepatterned substrate, wherein the controller module is configured to feedthe overlay compensation to the patterning tool to adjust at least oneparameter of the patterning tool associating with the forming of thepatterned substrate.
 3. The system of claim 1, wherein the identifyingof the set of outliers includes creating a wafer map on which theoverlay errors are depicted as vectors with magnitude and direction. 4.The system of claim 3, wherein the identifying of the set of outliersincludes dividing the wafer map into multiple fields and identifying theoutliers field-by-field.
 5. The system of claim 3, wherein theidentifying of the set of outliers includes detecting an abrupt changein the magnitude of the vectors with reference to a reference magnitude.6. The system of claim 1, wherein the creating of the set of noiseincludes dividing the overlay errors into multiple fields and applyingat least an inter-field filtering to the overlay errors.
 7. The systemof claim 6, wherein the fields are based on an exposure field schemeduring a lithography process in forming the patterned substrate.
 8. Thesystem of claim 6, wherein the inter-field filtering includes applying acorrection-per-exposure (CPE) model.
 9. The system of claim 1, whereinthe generating of the overlay compensation further includes: identifyingand removing extra noise from the set of filtered overlay errors, priorto the calculating of the overlay compensation.
 10. The system of claim9, wherein the identifying of the extra noise includes applying aninter-field filtering to the set of filtered overlay errors.
 11. Asystem for overlay control, the system comprising: a metrology toolconfigured to measure overlay errors associated with patterns formed ona wafer; and a controller module configured to read the overlay errorsfrom the metrology tool and generate an overlay compensation, whereinthe generation of the overlay compensation includes: identifying noisefrom the overlay errors by applying at least two filtering operations;grouping the overlay errors that are not noise identified by applyingthe at least two filtering operations into a set of filtered overlayerrors; and calculating the overlay compensation from the set offiltered overlay errors.
 12. The system of claim 11, wherein theidentifying the noise from the overlay errors includes dividing theoverlay errors into multiple fields, and wherein the at least twofiltering operations are selected from a ranking model applied to eachfield individually, an intra-field correction model applied to eachfield individually, and an inter-field correction model applied to themultiple fields collectively.
 13. The system of claim 12, wherein theidentifying of the noise from the overlay errors includes: determiningoutliers from the overlay errors by applying the ranking model;determining overlay errors due to reticle effects by calculatingresidues according to a difference between a magnitude of each overlayerror and a sum of a first value generated by the inter-field correctionmodel and a second value generated by the intra-field correction model;and identifying the outliers that are not overlay errors due to reticleeffects as noise.
 14. The system of claim 12, wherein the inter-fieldcorrection model is a correction-per-exposure (CPE) model and theintra-field correction model is an intra-field high order processcorrection (iHOPC) model.
 15. The system of claim 11, wherein thegeneration of the overlay compensation further includes: removing extranoise from the set of filtered overlay errors by applying anotherfiltering operation that is different from the at least two filteroperations, prior to the calculating of the overlay compensation.
 16. Amethod for overlay monitoring and control, the method comprising:collecting an overlay error set from a wafer patterned by a lithographysystem, wherein the overlay error set includes a plurality of overlayerror subsets according to a field scheme of the wafer; determiningoutliers from each of the plurality of overlay error subsets by applyinga ranking model to the corresponding overlay error subset; identifyinginside each of the plurality of overlay error subsets the outliers thatare not due to reticle effects as noise inside the corresponding overlayerror subset; forming a filtered overlay error set by excluding thenoise identified inside each of the plurality of overlay error subsets;and generating an overlay compensation from the filtered overlay errorset, wherein the overlay compensation is to adjust at least oneparameter of the lithography system.
 17. The method of claim 16, whereinthe ranking model examines an abrupt change of magnitudes of overlayerrors in each of the plurality of overlay error subsets.
 18. The methodof claim 16, wherein the ranking model is applied to a selected numberof overlay errors but not all of the overlay errors in each of theplurality of overlay error subsets based on empirical knowledge.
 19. Themethod of claim 16, wherein the identifying of the noise inside thecorresponding overlay error subset includes computing residues based ona correction-per-exposure (CPE) model and an intra-field high orderprocess correction (iHOPC) model.
 20. The method of claim 19, whereinthe iHOPC model has an order selected from 2 to 5.